There are already known containers usually called trays and intended to receive liquid, free-flowing or semi-solid products, as is often the case with food products: yogurt, butter, cooked dishes, etc. Consequently, these containers must be sealed. Also, they must be rugged in order to withstand stresses associated with transportation and storage and of an esthetic and attractive appearance since they are designed to present the products to the public.
It is difficult to reconcile these requirements since cardboard cannot provide a seal, being hydrophilic, although this material is that best suited to obtain a high quality esthetic appearance. Man-made materials are easy to use and provide a seal, but are of mediocre appearance because they are difficult to print.
Thought has therefore been given to combining cardboard and a man-made material to combine the advantages specific to each of these materials, but this leads to certain problems, notably associated with fitting lids intended to enclose the packaged product without risk of leakage.
One solution to this problem is described in the document FR-A-2.480.708 which teaches the creation of a single-piece frame to constitute a plane and regular surface to which a lid can be applied and fixed without serious risk of leakage.
Because of changing trends in consumption, customers are ever more demanding and containers must be capable of ever better performance to withstand, without damage, extremely stressful treatment such as freezing, deep-freezing, reheating, cooking the food in the packaging, long-term conservation of taste and smell, protection against oxidation and aging, barrier-effect walls, vacuum packaging, controlled atmosphere packaging, etc.
At this time there is virtually no packaging design based on a principle enabling implementation in different forms according to customer requirements. It will be understood, for example, that it is unrealistic to expect to use a tub for vacuum packaging without any deformation of its walls.
These changing requirements have the effect of ruling out, for certain applications, containers with walls using cardboard on the outside since moisture attack arises not only from the inside but also from the outside (storage in a humid atmosphere, for example).
Until now the cardboard has essentially been protected against moisture or grease inside it by providing a film of man-made material on the inside of the cardboard and not on the outside. This is the case with the container described in the previously mentioned document FR-A-2.480.708 and that described in the document FR-A-2.576.881.